The present invention relates generally to current generators, and more particularly to a precision tracking generator for use in integrated circuits.
Current generators, such as current sources and current sinks, are widely used in integrated circuits (IC's) to provide operating currents and reference currents where appropriate. It is fairly common to apply both positive and negative supply voltages to the IC device, and to reference the substrate of the device to a negative supply voltage. In such devices, it is often desirable to establish a precision stable current source or sink from the negative power supply, but responsive to a reference voltage.
Prior art current generator designs are legion. Several designs include simple two and three transistor circuits. These circuits, while easy to implement, lack precision performance. Such designs are susceptible to variations in transistor beta, transistor junction voltage, temperature and power supply voltage. More complicated current generator designs involving PNP transistors, usually in a feedback loop, provide an improvement in performance but are limited to low frequency operation. Furthermore, these designs are inherently unstable if lateral PNP transistors are used. Undesirable peaking and ringing in the output current can occur due to a sudden change of the reference voltage. An additional problem found in current generator designs with multiple current outputs is that of coupling between output devices. A change in the load on one of the output devices may modulate the current output of the other output devices.
What is desired is a precision tracking current generator which is easy to implement, is insensitive to variations in transistor beta, transistor junction voltage, temperature and power supply voltage, tracks changes in the reference voltage at high frequencies without peaking or ringing in the output current, and, in a multiple output configuration, minimizes coupling between output devices.